Well testing is a fundamental instrument for the exploration and planning of hydrocarbon fields, as it is capable of offering a wide range of dynamic information on the reservoir-well system.
Furthermore, the data on the reservoir fluids which can be obtained through sampling during well testing are of great importance, particularly for explorative or appraisal wells.
Conventional well testing is a consolidated process in the oil industry, both from an operative and interpretative point of view.
The well is induced to supply from the level/reservoir to be tested. 2 or 3 drawdowns are normally effected, at increasing flow-rate steps. During each phase, the flow-rate of the hydrocarbons produced is maintained constant and measured at the separator. Following the supply phase, the well is closed (with a valve at the head or bottom of the well) and there is a pressure build-up.
Pressure and temperature measuring devices (P/T gauges) are used during the test, situated at the well bottom, generally slightly above the producing level. During a well test samples of the reservoir fluid are normally taken, both on the surface at the separator and at the well bottom with suitable sampling devices.
Conventional tests are effected in wells of the explorative/appraisal or development/production type, temporarily (DST string) or permanently completed.
In all cases in which the well is not connected to a surface line, once the hydrocarbons supplied during the production test have been separated at the surface, they must be suitably disposed of.
The hydrocarbons produced at the surface during the test are normally burnt at the torch. Carbon dioxide (CO2) and sulphuric acid (H2S), lethal for human beings even at very low concentrations (a few parts per million, ppm), can be associated with these. The presence of H2S in the hydrocarbons produced causes considerable safety problems during the test.
The oil produced can be stored in tanks (onshore or offshore), if there is the possibility of sending it to a nearby treatment center, or eliminating it with suitable burners. The gas is in any case burnt in the atmosphere. The volumes of hydrocarbons supplied during a well test can be important. The following table shows an example according to the type of hydrocarbon and test to be carried out:
Conventional testOil well100-1000 m3(Associated gas 100-1000 m3each m3 of oil produced)Gas well1-10 · 106 m3
In addition to safety problems, there are also environmental problems due to the emission into the atmosphere of combusted hydrocarbons products and the risk of spilling in the sea or protected areas.
Environmental and safety problems are becoming increasingly more important, also as a result of environmental regulations which are more and more sensitive and restrictive as far as emissions into the atmosphere are concerned. Kazakhstan and Norway are among the countries in which present environmental regulations impose zero emissions.
Well testing allows a description of the unknown “reservoir+well” system. The principle is to stimulate the “reservoir+well” system by means of an input (flow-rate supplied) and measuring the response of the system as an output (bottom pressure). The pressure and flow-rate measurements provide an indirect characterization of the system, through known and consolidated analytical models found in literature.
The main objectives of conventional well testing are:                sampling to define the reservoir fluids        evaluation of the reference pressure of the fluids (Pav) and reservoir properties (actual average permeability k and transmissivity kh)        quantification of the damage to the formation (Skin factor). This effect, due to both the local reduction in permeability around the well and to geometrical effects of the flow shape, is quantified by means of a non-dimensional number (Skin factor)        evaluation of the well productivity (Productivity index PI for oil wells—Flow equation for gas well)        evaluation of possible areal heterogeneity or permeability barriers.        